The overall objective is to develop and demonstrate a technique and associated instrumentation to monitor remotely the temperature distribution in and around deep-seated malignant tumors during hyperthermia therapy. This technique is based on the ultrasound interrogation of several small, phase-changing sensors (or targets) implanted around the tumor being treated. In Phase I, concept feasibility will be assessed through laboratory tests. The Phase I tasks include sensor material selection and target design, fabrication and testing in heated phantoms and animal tissue in vitro using a diagnostic ultrasound scanner. Phase I results will include an assessment of key system characteristics (accuracy, range, etc.) and optimum design parameters (interrogation frequency and target size). Subsequent work (Phase II) will include refinement of the test system and evaluation of system performance in vivo, first through animal studies and then through initial human trials.